1. Field of the Invention
The present invention relates to a development device, and more particularly, to a development device including a developer roller that generates an oscillating electric field under which charged toner particles jump across a development gap to develop an electrostatic latent image recorded on a photoconductive surface.
2. Discussion of the Background
In electrophotographic image formation, development devices are used to develop an electrostatic latent image recorded on a photoconductive surface with charged toner particles. Generally, an electrophotographic development device includes an open-ended developer housing defining a developer chamber that accommodates developer and/or toner particles, and a developer roller rotatably mounted in the developer housing. The developer roller has its outer circumferential surface partially accommodated within the developer chamber and partially facing a photoconductive surface through an end opening in the developer housing. The developer roller rotates so as to advance toner loaded on the circumferential surface from inside the developer chamber to a development gap or zone defined between opposed surfaces of the developer roller and the photoconductive surface having an electrostatic latent image recorded thereon. Charged toner particles are transferred from the developer roller to the photoconductive surface across the development gap, and adhere to the electrostatic latent image to develop it into a visible image.
A particular type of such electrophotographic development is so-called hopping development, which generates a flare or aerosol cloud of charged toner particles with an oscillating electric field so as to transfer toner to an electrostatic latent image across a development gap. In a typical configuration, the hopping development device employs a tubular developer roller with multiple thin electrodes extending longitudinally along the roller at regular intervals all around a circumference of the developer roller. When energized, these electrodes generate an oscillating electric field therebetween, under which charged toner particles hop or move repeatedly to and fro between adjacent electrodes. In the development gap, hopping particles jump close to the photoconductive surface, and eventually adhere to an electrostatic latent image due to an electrostatic attractive force emanating therefrom.
Owing to the reciprocating hopping motion liberating toner from the developer roller, hopping development can selectively transfer toner to an electrostatic latent image with an extremely low voltage (e.g., on the order of several tens of volts) between charged image areas and adjacent non-image areas. The result is a low-power development process design that compares favorably, at least in terms of power consumption, to a configuration that transfers toner across a development gap primarily based on a development bias or voltage applied between a developer roller and an electrostatic latent image.
One common problem with a development device in which the developer roller is accommodated in an open-ended housing is leakage of toner from the housing opening. That is, toner particles, stirred up within the developer chamber, leak through any clearance between the surface of the developer roller and edges of the housing opening. Such leaking toner results in contamination of areas adjacent to the development device as well as smudges on recording media (e.g., sheets of paper, etc.) passing through the contaminated surfaces during image formation.
To prevent toner leakage from an end opening in a developer housing, conventional development devices employ a cantilevered flexible film member or blade to seal the opening. Typically, the sealing blade has one edge supported on the edge of the housing opening and another edge contacting the circumferential surface of a developer roller. The contacting edge of the flexible blade prevents airborne toner from escaping from the developer chamber while allowing toner resting on the roller surface to pass therethrough to or from the developer chamber. Such weak sealing by a cantilevered flexible member effectively prevents toner leakage in a conventional development device that transfers toner across a development gap with an electrically biased developer roller.
Unfortunately, the conventional sealing technique is not compatible with a hopping development device described above. This is because hopping toner, which has little adhesion to the surface of a developer roller, readily migrates from the roller surface when brought into direct contact with a sealing member. Naturally, such migration of toner results in reduced efficiency of toner delivery to or from the development zone, causing various adverse effects on the performance of the image forming apparatus employing the hopping development device.
For example, a sealing blade provided to an upstream edge of the housing opening can remove substantial amounts of toner particles loaded for delivery to the development zone. This results in development deficiencies due to insufficient supply of toner in the development zone, even when the developer roller is loaded with proper amounts of toner inside the developer chamber. On the other hand, a sealing blade provided to a downstream edge of the housing opening can remove residual toner from the developer roller before the toner can return to the developer chamber. This results in toner particles accumulating on the sealing blade and eventually spreading out to contaminate areas adjacent to the development device.
These detrimental effects of a cantilevered blade sealing the clearance between the opening edges and the hopping developer roller could be alleviated by providing a narrower gap between the free edge of the sealing blade and the roller surface roller instead of directly contacting the blade edge and the roller surface. However, such a configuration is impractical because the alleviation is ineffective when the edge-to-surface gap is greater than the height to which toner particles jump from the roller surface.